class_docstr: GHI l1 file handler.
docstr: Get the area definition.
docstr_tok: ["Get","the","area","definition","."]
code: def get_area_def(self, key):
      """Get the area definition."""
      # Coordination Group for Meteorological Satellites LRIT/HRIT Global Specification
      # https://www.cgms-info.org/documents/cgms-lrit-hrit-global-specification-(v2-8-of-30-oct-2013).pdf
      res = key["resolution"]
  
      pdict = {}
  
      c_lats = self.file_content["/attr/Corner-Point Latitudes"]
      c_lons = self.file_content["/attr/Corner-Point Longitudes"]
  
      p1 = (c_lons[0], c_lats[0])
      p2 = (c_lons[1], c_lats[1])
      p3 = (c_lons[2], c_lats[2])
      p4 = (c_lons[3], c_lats[3])
  
      pdict["a"] = (
          self.file_content["/attr/Semi_major_axis"] * 1e3
      )  # equator radius (m)
      pdict["b"] = (
          self.file_content["/attr/Semi_minor_axis"] * 1e3
      )  # equator radius (m)
      pdict["h"] = (
          self.file_content["/attr/NOMSatHeight"] * 1e3
      )  # the altitude of satellite (m)
  
      pdict["h"] = pdict["h"] - pdict["a"]
  
      pdict["ssp_lon"] = float(self.file_content["/attr/NOMSubSatLon"])
      pdict["nlines"] = float(self.file_content["/attr/RegLength"])
      pdict["ncols"] = float(self.file_content["/attr/RegWidth"])
  
      pdict["scandir"] = "S2N"
  
      pdict["a_desc"] = "FY-4 {} area".format(
          self.filename_info["observation_type"]
      )
      pdict[
          "a_name"
      ] = f'{self.filename_info["observation_type"]}_{res}m'
      pdict["p_id"] = f"FY-4, {res}m"
  
      proj_dict = {
          "a": pdict["a"],
          "b": pdict["b"],
          "lon_0": pdict["ssp_lon"],
          "h": pdict["h"],
          "proj": "geos",
          "units": "m",
          "sweep": "y",
      }
  
      p = Proj(proj_dict)
      o1 = p(p1[0], p1[1])  # Upper left
      o2 = p(p2[0], p2[1])  # Upper right
      o3 = p(p3[0], p3[1])  # Lower left
      o4 = p(p4[0], p4[1])  # Lower right
  
      deller = res / 2.0
  
      area = get_area_definition(
          pdict, (o3[0] - deller, o4[1] - deller, o2[0], o1[1])
      )
  
      return area
  
file_path: satpy/satpy/readers/ghi_l1.py
imports: import logging
  from pyproj import Proj
  from satpy.readers._geos_area import get_area_definition
  from satpy.readers.fy4_base import FY4Base
label_desc: Use image node_id 4 for calling the HDF_GHI_L1 obj's underlying member method code with example usage: obj.get_area_def(key) and returns: area